Radiation therapy remains the single most effective treatment for malignant brain tumors, but in many cases, toxicity to normal brain impedes therapeutic doses sufficient for local control to be achieved. A substantial effort has been directed toward overcoming the unfavorable side effects of brain tumor radiation therapy. Small animal studies in our Branch indicate that pentobarbital anesthesia during cerebral irradiation reduces the toxicity of the ionizing radiation. Although mechanisms of this phenomenon remain unclear, it may arise from suppression of brain synaptic activity or metabolism. After baseline MRI scans of the brain and neuroendocrine testings, primates (Macaca mulatta) underwent whole brain X-irradiation in 10 daily fractions, 360 rads each (total dose of 3600 rads). The monkeys in the study group were anesthetized with pentobarbital during the irradiation, whereas the animals in the control group received ketamine. Each group consists of 6 animals. Neuroendocrine testing and MRI scan follow-up studies are performed at 3, 6 12, 18 and 24 months after irradiation to detect potential differences in the degree of brain injury in the two groups.